1. Field of the Invention
The present invention relates to printing plates comprising a substrate and a radiation-absorptive layer, wherein the radiation absorptive layer comprises at least one modified pigment product.
2. Description of the Related Art
Printing plates are used in several areas of image reproduction, including lithographic printing (also known as offset or planographic printing), flexographic printing, and gravure printing (also called intaglio or rotogravure). In general, the printing process involves the development of an image on the plate followed by exposure to an ink.
Lithographic printing plates are among the most widely used for making printed copies. Generally, an infrared or near-infrared laser-imageable lithographic printing plate includes at least the following layers: paper, a grained-metal, or polyester plate or sheet-like substrate and a radiation-absorptive layer coated thereon. Protective layers for the substrate or the surface of the coated plate may also be used. When coated onto the substrate, this protective layer can also serve as an adhesion-promoting primer. Other layers may be used, for example, to improve adhesion between layers and durability of the printing plate.
In general, the radiation-absorptive layer comprises a photothermal conversion material capable of interacting with the imaging radiation and a polymeric resin or binder. In the imaging process, regions of the plate are selectively exposed to a laser output or other heat source capable of removing or chemically modifying the radiation-absorbent layer. Typically, heat sensitive lithographic printing plates are exposed to radiation having wavelengths of between 800 and 1200 nm. The laser output will define a pattern on the printing plate and either remove or chemically or physically modify only those regions of the radiation-absorptive layer which define the pattern. Afterwards, the printing plate can be further developed by subjecting it to a solvent capable of removing the exposed region(s), if any remains, which define the pattern or, if desired, the plate can be developed such that the non-exposed region(s) are removed. The details of the various conventional components and techniques for such printing plates are described in U.S. Pat. No. 5,493,971; U.S. Pat. No. 5,705,308; EP 0 803 771 A1; EP 0 770 494 A2; EP 0 770495 A1; as well as PCT Publications WO 96/20429 and WO 98/31550 and the patents set forth therein, all of which are incorporated in their entirety by reference herein.
Several types of polymers have been used in the radiation-absorptive layer. Representative polymers include polyurethanes, poly(vinyl alcohol), polyacrylates, polystyrene, styrene-acrylate polymers, metal oxide polymers, epoxy resins, and phenolic polymers. In addition, phenolic polymers have long been shown to be useful in photoresist applications.
The photothermal conversion material can be either a pigment or a dye. For example, UV- and IR-active dyes have been disclosed in phenolic printing plate applications (see DBP 879205 and WO 97/39894). IR-absorptive pigments such as carbon black have also been shown to be useful in a lithographic printing plate (see, for example, WO 99/08157, WO 96/20429, WO 99/11458, and U.S. Pat. No. 6,060,218 in which carbon black is present in a phenolic polymer).
Carbon blacks that have been modified to have carboxylate or sulfonate functionalities have also been disclosed for use in lithographic printing plates. For example, see WO 99/04974, WO 99/19143, WO 99/19144, WO 99/37482, and WO 99/37481. However, none of these disclose the use of these modified carbon blacks in a phenolic polymer nor do they disclose the use of other modified pigment products.
Other materials have also been studied as photothermal conversion materials in phenolic polymers. For example, in the photoresist area, soluble compounds containing an S═O or C═O bond have been shown to act as dissolution inhibitors of phenolic resins (Yan and Reiser, Macromolecules 1998, 31, 7723). When a phenolic polymer composition containing a photothermal conversion material bearing such a group is irradiated with UV light, the hydrogen bonding network of the phenol groups is disrupted. The irradiated regions thus become soluble in alkaline developing solutions.
PCT Publication WO 00/16987 discloses an imaging member comprising at least one heat-sensitive polymer and a photothermal conversion material, such as a dye or pigment. They polymer is capable of undergoing a transformation from a hydrophilic to a hydrophobic state, or vice versa. No modified pigment product, particularly those that can undergo a chemical transformation, is disclosed.
Pigments such as carbon black are broad band radiation absorbers and, as such, offer an improvement in performance over dyes. However, the effectiveness of pigments such as carbon black as a photothermal conversion material in a printing plate is dependent on the dispersibility of the pigment in the polymer. Thus, there is a need for printing plates comprising pigments such as carbon black with improved dispersibility in polymers used to produce printing plates.